This invention relates to a printing ink with an IR-absorbent additive, a semifinished product for producing it and a document printed with said ink.
Data carriers, such as documents, in particular documents of value, such as identification cards, airplane tickets, passports, bank notes, papers of value or the like, and other objects of value are provided with security features for protection against forgery or falsification. Two groups of security features are essentially distinguished, the first group including so-called xe2x80x9chuman featuresxe2x80x9d designed so that the authenticity of the documents or objects of value can be recognized by mere sensory perception. This group includes in particular providing optically variable inks on the documents or objects of value and applying holograms.
In a second group including so-called xe2x80x9cmachine features,xe2x80x9d features are assigned to the documents of value which can usually be detected by machine, i.e. only with the aid of sensors. In the paper-of-value field, in particular magnetically or electrically detectable features have become established. Moreover, optically recognizable markings of security documents are also known whereby the wavelength with which the sensor can detect the marking is outside the visually perceptible spectral region, e.g. in the infrared (IR).
With the sensors commonly used today one detects such an IR-absorbent marking by illuminating the document with an IR-emitting light source and measuring the diffuse reflection, i.e. the light of the irradiation wavelength emitted by the document or marking by reason of this illumination. A document or printed marking is deemed IR-absorbent if it falls below a certain, fixed diffuse reflection value. A threshold as of which a document or marking is deemed IR-absorbent can be fixed as a diffuse reflection value of 50% for example.
In order to reach or fall below the aforementioned diffuse reflection values, a document must have a certain quantity of IR-absorbent additive, e.g. in one of its printing inks.
For marking data carriers GB 1 534 403 discloses a security document printed with two inks appearing identical in the visible spectral region but differing in their infrared-absorbent behavior. For this purpose carbon black is added to a pigmented ink as an IRI-absorbent additive for example. However the addition of carbon black to inks makes them appear darker or xe2x80x9cdirtier,xe2x80x9d while it is necessary to add so much carbon black to the ink for sufficient marking thereof that darkening or xe2x80x9cdirtyingxe2x80x9d of the ink is unavoidable with the use of carbon black as an IR-absorbent additive, and the ink is substantially impaired in its visible color effect. The direct result is that only a very limited color range of IR-absorbent inks can be produced, this color range including substantially the so-called xe2x80x9cdirty brownxe2x80x9d tones.
In order to avoid this disadvantage EP 0 553 614 A1 has proposed using so-called phthalocyanines, which are substantially transparent or hardly visible, rather than carbon black as an IR-absorbent additive. However these phthalocyanines have the disadvantage that they must be produced by a complex chemical process.
The problem of the present invention is accordingly to propose a printing ink having an IR-absorbent additive which is inexpensive and readily available and has only little influence on the ink in the optical spectral region.
The invention is based on the finding that carbon black and graphite particles, which are both built up from elementary carbon, have almost the same color effect in the visible spectral region. Moreover, measurements in the IR show that the two kinds of particles hardly differ in their absorbent behavior in the IR. However it has surprisingly turned out that the visual influence of an ink by the addition of graphite to this ink is clearly lower than the influence resulting from addition of the same quantity of carbon black. In consideration of the fact that an increase in the quantity of IR-absorbent additive leads directly to increased IR absorption of the ink, this finding means that much more graphite can be added for a given tone than would be possible using carbon black as an IR-absorbent additive, so that by using graphite as an IR-absorbent additive one can produce considerably higher IR absorption for a given tone than using carbon black.
The tones to be provided on a document are usually given and contain no IR-absorbent additives. If this tone is to be produced at least partly with IR-absorbent properties, the inventive additive makes it possible to adjust this given tone by developing a new ink formulation containing the inventive IR-absorbent additive in percentages by weight of less than 7%, but preferably less than 5%, based on the total weight of the ink. In a particularly preferred embodiment the inventive ink contains less than 3 wt % of the additive. By adding graphite in the stated percentages one can produce inks having an IR diffuse reflection of less than 50% and therefore deemed IR-absorbent for the sensor used.
Although it is fundamentally possible to produce the IR-absorbent ink by adding graphite powder to an ink directly in the desired quantity, it has proved advantageous first to produce a xe2x80x9cpastexe2x80x9d as a semifinished product in which the graphite powder is already mixed homogeneously in a binder. For producing this paste one mixes or disperses the graphite powder for example together with a binder-containing transparent or opaque white color and a thinner in a mixer or dispersing apparatus until a paste arises in which the graphite powder is distributed homogeneously. According to another method the graphite powder can also be processed into a homogeneous paste by mixture or dispersion directly in a suitable binder, optionally with the addition of solvent.
The paste containing IR-absorbent graphite can be used directly for producing IR-absorbent ink or stored for later use. In production of the ink the desired proportions of paste are mixed with a basic color and optionally further proportions of transparent or opaque white color so as to yield the IR-absorbent ink.
The thus produced ink can be used for printing documents or other objects of value in order to provide them with an IR-absorbent feature substance.